It has been determined that the sex of offspring is controlled by the chromosomes of the particular spermotozoon or sperm cell which fertilizes the egg. More specifically, some of the spermatozoa (hereinafter called "sperm") are genotypically known to contain X chromosomes, which carry female producing genes, while the others contain Y chromosomes, which carry male producing genes. In microscopes, the X chromosomes appear larger in size than the Y chromosomes. When a sperm containing X chromosomes (hereinafter called X-sperm) combines with the egg (which contains X chromosomes), female offspring results. When a sperm containing the Y chromosomes (hereinafter called Y-sperm) combines with the egg, male offspring results. The sperm population in an ejaculate of a mammalian male contains both X-sperm and Y-sperm. Heretofore, separation of these sperm in X and Y components has not been satisfactorily achieved. It is evident, however, that a satisfactory procedure for separating the two kinds of sperm, to isolate substantially pure X and Y-sperm fractions, would permit a choice or selection of the ultimate sex of the offspring.